GEOLOGY of central wales. 155 



between Aberystwyth and Plynlimmon — namely, (1) the principal 

 axis of Aberystwyth, (2) the principal axis of Plynlimmon, and (3) 

 the two minor axes running (a) from Tstrad Meurig through Llan- 

 trisant and Cwm Symlog, and (b) along the gorge of the liheidol 

 about Pont Erwyd and the Devil's Bridge. The further northward 

 extension of (a) is probably indicated by the grits of Yr Carreg and 

 lloel y Llyn. These minor folds have not been recognized further 

 south ; and, indeed, we might expect them to disappear as the lateral 

 spread of the beds becomes restricted. 



At Lampeter the well-known olive shales and grits with Nereites 

 &c. are exposed in quarries. The grits are in thin beds (2-4 inches), 

 hard and laminated, with partings of shale either light (olive) or 

 dark in colour ; no cleavage is seen. The fossils are : — 



!N"ereites Sedgwickii, Murch. Neniertites Olivantii, Murch. 



■ canibrensis, Murch. Palaeockorda tardifurcata, Keep. 



Myrianites M'Leayi, Murch. Retiofucus extensus, Keep. 



also other worm-like and algoid markings. Altogether this is a 

 pretty typical metalliferous slate, though more arenaceous than is 

 common. 



Fig. 3. — Railway-cutting between Traws Coed and Caradoc 

 Waterfall, 1877. 



Contortions. — The woodcut (fig. 3), representing a railway-cutting 

 east of Traws Coed, illustrates some of the frequent and sharp contor- 

 tions of this series. All the perceptible foldings, however, numerous 

 and striking as they are, are far from adequately showing the real 

 complexity of the infinite twistings and inversions of this series, the 

 existence of which is proved principally by considerations from the 

 fossil evidences. Without fossil evidences it is, as a rule, impossible 

 to detect inversions ; but still, in those places where grits with 

 fucoidal markings &c. occur, we have another key to the true rela- 

 tions of the rocks — namely, in observing the positions of the curious 

 prominences upon the surface of the grit, these being normally 

 always upon the under surfaces ; but where the beds are inverted 

 (as at IForben, Machynlleth, &c.) they are found upon the upper sur- 

 faces, the true worm-trails (Nereites &e.) being in this case impressed 

 upon the under surfaces, in inverted position. 



These inversions explain away in great degree the enormous 

 apparent thickness (miles of strata) of this series ; but a very great 

 vertical series still remains well attested in actual steep and preci- 



